1. Field of the Invention
The present invention relates to a novel acrylic ester copolymer plastisol composition. More particularly, the present invention relates to a novel acrylic ester copolymer plastisol composition having excellent storage stability even when generally used inexpensive plasticizers are used, giving a molded sheet showing no bleeding and generating no toxic gas on heating to a high temperature, on burning or in case of fire.
2. Description of the Prior Art
A plastisol is a viscous composition of a liquid or paste form which is prepared by homogeneously dispersing polymer particles having a specifically adjusted particle size (for example, an average size of primary particles of 0.05 to 2 .mu.m) and a specifically adjusted particle size distribution in a liquid substance comprising a plasticizer as the major component and other components added according to necessity, such as diluents and liquid stabilizer, together with other powder materials utilized according to necessity, such as pigments, fillers, thixotropic agents, blowing agents, powder stabilizers and the like. As the polymer particles, particles of polyvinyl chloride, such as homopolymer of vinyl chloride, copolymer of vinyl chloride with vinyl acetate and the like, are generally utilized.
The polyvinyl chloride plastisols have excellent storage stability at room temperature for a long time and form a homogeneous gel by heating to give soft sheets and coated layers having high tensile strength with ease. They are widely utilized in the field of coating of steel plates, building materials, clothing materials, daily necessary goods, sundries, automobile parts and the like.
Polyvinyl chloride has good mechanical properties, handling, cold resistance, transparency, foaming property and flame retarding property and is inexpensive. However, it has a problem that hydrochloric acid is generated on heating to a high temperature, on burning or in case of fire and is not a desirable material from the point of view of safety, health and environmental protection.
For improving the problem, plastisols comprising a polymer of alkyl (meth)acrylate and an ester plasticizer as the essential components thereof have been proposed (Laid Open Japanese Patent Applications Showa 51-71344 and Showa 52-42590, which correspond to English Patent 1516510 and U.S. Pat. No. 4,071,653, respectively).
However, even though the homopolymers and copolymers of alkyl (meth)acrylates do not have the problem of polyvinyl chlorides described above, they have a problem that, when a plastisol is prepared by dispersing the polymer of alkyl (meth)acrylate in a general use plasticizer, such as di-2-ethylhexyl phthalate (DOP) which is generally utilized for polyvinyl chlorides, and a sheet is prepared by heat gelation of the plastisol, the sheet inevitably shows a large extent of bleeding when it is cooled to the room temperature. To avoid the problem of bleeding, an expensive plasticizer, such as tricresyl phosphate (TCP), butyl benzyl phthalate, acetyl tributyl citrate or the like, must be utilized for preparation of the sheet. These plasticizers have another problem in that they have high polarity and the polymers are swollen with the plasticizer to cause remarkable increase of viscosity of the plastisol during storage.
Compatibility of the polymers of alkyl (meth)acrylates with the general use ester plasticizers can be improved by using alkyl acrylates or alkyl methacrylates having a bulky alkyl group for copolymerization (Japanese Patent Publication Showa 62-3868 and U.S. Pat. No. 4,199,486). However, plastisols prepared according to this method have a problem that viscosity is remarkably increased during storage, leading occasionally to gelation at the room temperature and, therefore, they are not advantageous for practical use.
In general, the storage stability of plastisol tends to be improved by increasing the size of the dispersed particles. When the size of the dispersed particles is increased, the gelation property of the plastisol is generally decreased, leading to unfavorable properties, such as deteriorated appearance of heat molded sheets and decrease of mechanical strengths.
A plastisol having good storage stability of viscosity was disclosed which utilizes resin particles prepared by three stages: the first stage polymerization of forming particles of homopolymer or a copolymer of methyl methacrylate, the second stage polymerization of adding a polymerizable carboxylic acid to the polymer formed and the last stage of adding a salt-forming agent (Japanese Patent Publication Showa 55-35420. This plastisol has problems that multiple stages of polymerization are essential and production efficiency is decreased and that molded products show bleeding unless an expensive highly polar plasticizer such as TCP is utilized.
A copolymer of a micro particle form prepared by reaction of a copolymer comprising carboxyl group with a polyfunctional base substance was proposed as resin particles giving plastisol having good storage stability (Laid Open Japanese Patent Application Showa 63-95248 and Laid Open German Patent Application 3713462). This copolymer has a problem that the applicable plasticizers are limited to those having high polarity.